there is an awful lot that goes into making a bridge... and you must possess the correct programs and skill to do it... in all of BC's history, only a handful of people have been able to figure it all out from beginning to end, which is why we don't have more bridges than we do...
it takes alot of time, effort, patience, etc etc; be warned about that first...
there is modeling, texturing, lightmapping, animating, scripting, and other things that are involved also, so you must have keen sense and skill about how BC works before even thinking of making a new bridge from scratch...
i dont mean to sound pessimistic, but all too often people think making bridges must be simple and then once they realise just how complex and complicated it really is, they give up... it is a major task, building a bridge and getting it to work ingame...